1. Technical Field of the Invention
The present invention relates to modulating and demodulating devices (modems) and, in particular, to a modem supporting the transmission of data communications over both an analog and a digital cellular telephone air interface as well as a conventional analog wireline telephone interface.
2. Description of Related Art
Cellular telephone networks support a plurality of calling services. The most commonly recognized and widely used calling service relates to the handling of voice communications to and from the mobile stations of cellular subscribers. Cellular telephone networks further support the provision of data calling services. For example, subscriber asynchronous data communications and facsimile communications are also supported by the cellular telephone network.
Cellular telephone networks utilize a number of different types of air interfaces for handling radio frequency communications between a mobile station and a base station. Historically, cellular communications, both voice and data, have been effectuated over an analog air interface using an analog traffic channel. For data communications, this typically necessitated the placement of a modem between the data terminal equipment and the subscriber mobile station to enable digital data signal transmission over an analog traffic channel of the analog cellular air interface. Recently, however, new capabilities have been provided in implementing a digital air interface for cellular service (see, for example, the TIA/EIA IS-136 Specification). The availability of a digital air interface advantageously obviates the need for implementing the modulation and demodulation functionalities at the mobile station when engaging in a digital data communication. The data terminal equipment in such cases is then connected directly to the mobile station, with the digital data signal carried over a digital traffic channel of the digital cellular air interface (see, the TIA/EIA IS-130 and 135 Specifications).
There may exist instances within the cellular telephone network where a digital traffic channel on the digital air interface may not be available for use. For example, all digital traffic channels may at the time of mobile station access be in use handling the communications of other cellular subscribers. Alternatively, the particular part of the cellular network being accessed may not yet have been upgraded to support digital traffic channels. In such instances, for the subscriber desiring to immediately make or continue with a cellular data communication, a modem must be used between the data terminal equipment and the mobile station. Such a modem, however, should be flexible enough in design to support data communications over a conventional wireline telephone network as well as both the cellular analog and digital air interfaces when necessary and available.